Possible solution to your dilemma (Score 1)

You may be able to rig your phone system (at work) or buy/lease a new Phone system with a powerful Call Blocking Feature. First buy a list of nationwide headhunters from a Marketing List Broker (couple hundred bucks, probably - insist on getting the most freshly edited and compiled lists available - this occupation changes faces intensively frequently.) and feed in all the main phone numbers and even the NAMES of headhunters (those are for sale too) in the firms they are associated with - for automatic restriction - and simply prevent them from gaining phone access into your office. Another thing you need to do is at the very least have your phone lines become transparent to everyone, so names and numbers are all visible, by the users while calls come in - they should be interactively visible and on/operational. Again the idea is to quickly and simply IDENTIFY them so you and feed them into you caller restricted Database. Have managers pore over these lists of suspicious incoming callers and their phone numbers, and question these employees about them. Also, I'm sure your employees may be equally annoyed with this endless stream of aggressive, nosy morons trying to pry information out of them from their home or cells lines. You must toil now and really invest the time to make these seekers names and phone numbers completely transparent. Sleazy recruiters are likely to play dirty pool in their attempts to shake your agency down to it's roots, making many people feel highly unsettled and uncomfortable in the process, so you may very well be able to mobilize and otherwise solicit your employees to help rid them of this disease, by allowing them to identify to you, one at a time, all the those rogue phone numbers/AND names they're getting from these vapid recruiters, and simply stop them with some help from your own overwhelmed and exasperated people. You can announce an aggressive internal security program being put in place to address this crisis, letting these sleazebuckets as well as your own people to know that these outside recruiters are fighting a losing battle. Tell them you will, one phone line at a time, narrow down these callers and will simply suppressed them from being able to generally solicit anyone into your firm. See if you like this idea! If headhunters want to ply their trade and attack your ranks, let them do it at least off company time, and at significant tactical disadvantages.